The invention relates to microelectrodes that find applications in many electrochemistry areas; more particularly, the invention relates to a composite wire microelectrode that finds application in mapping variations in current densities when used in scanning techniques to analyze corroded metal surfaces.
A well known method of analyzing corrosion of a metal surface is to scan a fine-tipped microelectrode sensor over the surface while maintaining the sensor tip at a fixed distance above the surface. The sensing probe measures variations in current densities, which depend on potential gradients developed in an electrically conductive solution. Differences in behavior of the scanned surface lead to the differences in current flow. Variations in the sensed electric potential above the scanned surface are a result of IR-drops in the solution, thus those variations reflect changes in current density along the path scanned by the microelectrode sensing probe. Such scanning techniques have been used to study a wide range of corrosion problems including measuring stress corrosion cracking, pitting corrosion, intergranular corrosion, biocorrosion and the use of corrosion inhibitors in solution or applied to a metal surface.
In order to improve the accuracy of the current density measurements obtained with such scanning techniques, it is desirable to use a microelectrode sensor probe that has a very fine tip or a tip with a very small radius of curvature. In the past such fine-tipped microelectrode probes have been produced by commonly known electrolytic polishing methods wherein a thin wire sensor probe is dipped in an electrolyte while the wire and electrolyte are subjected to a voltage, thereby to effectively polish the sensor tip of the wire to a small radius of curvature in the range of 1 .mu.m. Following tip production by such methods, the surface of the wire must be coated to insulate all but the tip of the wire which senses the potential in solution.
Another problem inherent in the use of previously known scanning techniques to measure corrosion is that the fine sensor tips of microelectrode sensor probes must be renewed frequently during use in order to maintain them in suitably sharp condition for performing the desired sensor functions. In some cases, for example as shown in U.S. Pat. No. 5,002,651, a renewable tip is achieved by simply breaking the end off of a microelectric probe when it becomes contaminated. Such an approach is generally not suitable for the fine wire microelectrode probes required for applications in corrosion measuring scanning techniques, because the very thin probe wire would be unsuitably weak in the absence of some auxiliary supporting structure. Other patents, such as U.S. Pat. Nos. 4,427,483 and 4,452,249, describe electric probes that are coated with metal which acts as an electrically conducting shield to reduce electrical noise during application of the probe. Of course, such metal shields help mechanically support the thin probe. In the applications described in those patents, the shielding must be present as close to the probe tip as possible and must also electrically isolate the remainders of the metal wire from the medium into which the tip is inserted. In the application described in those patents the metal tip must be removed and coated and those processes are described in the respective patents, with a wax coating subsequently being applied to the probe tip. The use of such a wax coating, or the use of various polymer coatings as is taught in other prior art techniques, fails to make the coatings scratch resistant, accordingly, such coated probes, can be damaged during use. For example, U.S. Pat. No. 3,436,329 describes a silica support that is coated with multi-coatings of metals and insulators. Again, those coatings are thin and fragile and the coated tip is not renewable.